Laser printer with piezoelectric apparatus to reduce banding by adjustment of a scanned laser beam

ABSTRACT

A print apparatus includes a photoconductor and a mechanical system for moving the photoconductor past a scan line exposure station. The print apparatus includes a signal generator for providing outputs indicative of the movement of the photoconductor. A first comparator produces a first position error signal that is derived from a difference between a reference signal and a position error signal output, such position error signal indicating that the position of the photoconductor differs from a predetermined print position that is determinable with respect to the reference signal. The print apparatus further includes a laser beam scanner and a beam detector for producing a scan position signal. A second comparator is responsive to the scan position signal and the position error signal from the first comparator to produce a beam deflection control signal that is applied to a beam deflector having a mirror attached to a piezoelectric bimorph crystal cantilever element. The beam deflector moves the laser beam in a direction which reduces the beam deflection control signal and position the laser scan at a position closer to a predetermined print position.

This application is a continuation in part application and claimspriority from application Ser. No. 08/621,053 filed Mar. 22, 1996, nowU.S. Pat. No. 5,760,817, which is a continuation of application Ser. No.08/262,405 filed Jun. 20, 1994, now abandoned.

FIELD OF THE INVENTION

This invention relates to laser print mechanisms and, more particularly,to a system for reducing "banding" which occurs when print scan lines ina laser printer are subject to positioning errors.

BACKGROUND OF THE INVENTION

Banding is a term used to define alternate streaks of light and darkareas (or bands) that are produced on an output sheet in a laserprinter. These bands are produced as a result of non-constant placementof print scan lines. Dark bands are produced when the scan lines arecloser together and lighter bands are produced when the scan lines arefarther apart.

In a laser printer, pitch variation between scan lines can occur as theresult of a number of system errors. The major cause is speed variationsin the moving photoconductor. Those speed variations occur due toimperfections in the mechanical parts that comprise the photoconductordrive system. These imperfections may include out-of-round parts, gearinaccuracies, bearing runouts, mechanical coupling errors, etc.Structure vibration states which result from mechanical resonances canalso cause relative motion between a laser scanner and thephotoconductor and result in a banding affect on output sheets.

In general, banding is not apparent to the user when documents containonly text. However, because the resolution available from laser printershas seen substantial improvements, such printers are now employed toprint full grey scale graphics images. Banding is clearly visible insuch images.

When banding is apparent the extent of misalignment may exceed severalscan line widths. In a monochrome printer, scan line misalignment isusually less than plus or minus up to one scan line width. For a colorprinter, banding is aggravated by misregistration of color planes inaddition to scan line misalignment. A color printer has several colorplanes, for example, black, yellow, cyan, and magenta. Misregistrationbetween planes can be several scan line widths.

The prior art has attempted to solve the banding problem by specifyingmore accurate (and more expensive) parts for the laser printer.Vibration problems have been reduced through provision of stifferstructures that are usually heavier and more expensive. Further,increasingly more sophisticated (and expensive) speed controls for drivemotors have been implemented so as to assure relatively constantphotoconductor speeds. Nevertheless, for laser printers that employphotoconductive belts, such improvements still do not eliminate banding.This is due to the fact that the belts themselves evidence some stretchand positional modification when subjected to impulse loading. Afeedback system which attempts to correct for mechanical movements ofthe photoconductor and/or drive system cannot respond fast enough tototally overcome the banding effect. While high bandwidth servo systemsapproach a solution to the problem, they are still confronted with theproblem of altering speed and/or position of mechanical elements whichhave both inertia and momentum that tend to negate the immediatecorrection actions. Such mechanical elements also exhibit a "resiliency"that tends to limit the responsiveness of the system to high frequencycorrection actions.

Accordingly, it is an object of this invention to provide an improvedsystem for reduction of banding in a laser printer.

It is a further object of this invention to provide a banding reductionsystem in a laser printer wherein few additional mechanical parts arerequired.

It is yet another object of this invention to provide a bandingreduction system for a laser printer wherein the banding reductionaction is not dependent upon a high bandwidth servo system forcorrecting printer mechanical movements.

SUMMARY OF THE INVENTION

A print apparatus includes a photoconductor and a mechanical system formoving the photoconductor past a scan line exposure station. The printapparatus includes a test signal generator for providing outputsindicative of the movement of the photoconductor. A first comparatorproduces a first position error signal that is derived from a differencebetween a reference signal and a position error signal output, suchposition error signal indicating that the position of the photoconductordiffers from a predetermined print position that is determinable withrespect to the reference signal. The print apparatus further includes alaser beam scanner and a beam detector for producing a scan positionsignal. A second comparator is responsive to the scan position signaland the position error signal from the first comparator to produce abeam deflection control signal that is applied to a piezoelectric beamdeflector. The deflector moves the laser beam in a direction whichreduces the beam deflection control signal and positions the laser scanat a position closer to a predetermined print position.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a laser printer that incorporates afirst embodiment of the invention.

FIG. 2 is a side sectional view of a galvanometer-operated mirrorassembly employed to reposition a laser beam in accordance with aposition error signal derived from mechanical drive components withinthe printer.

FIG. 3 is a block diagram of electronic circuits employed in the firstembodiment of the invention shown in FIG. 1.

FIG. 4 is a perspective view of a laser printer showing a secondembodiment of the invention.

FIG. 5 is a block diagram of electronic circuits utilized to implementthe second embodiment shown in FIG. 4.

FIG. 6 is a side sectional view of a piezoelectric beam deflector in oneembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates principal components of a laser printer thatincorporates the invention hereof. A photoconductor belt 10 passes overan idler roller 12 and a drive roller 14. Drive roller 14 is operated bya drive motor which, in turn, transmits its rotary action through a geartrain to drive roller 14. (The drive motor and coupling gear train arenot shown.) Photoconductor belt 10 passes over a laser write platen 16which has an optical sensor 18 mounted at one extremity. A magneticsense head 20 is positioned at another extremity of laser write platen16 and senses signals that are generated when a magnetic synch track 22passes in contact therewith. Magnetic synch track 22 is recorded on amagnetic track along the interior surface of photoconductor belt 10 andprovides synchronizing signals to enable positional detection ofphotoconductor belt 10 in relation to a reference clock frequency.

A laser assembly 24 directs a laser beam 26 at a galvano-motor-mirrorassembly 28 (hereafter referred to as "galvano") which reflects laserbeam 26 towards a rotating faceted mirror 30. The rotation of facetedmirror 30, in the known manner, scans laser beam 26 along a path betweenscan extremity lines 32 and 34. The scanned laser beam passes through alens 36 and is reflected by an elongated mirror 38 to create a rasterscan line 40 on photo conductor belt 10 directly over laser write platen16. To implement this invention, the scanned laser beam is caused toscan past the edge of photoconductor belt 10 and to intercept opticalsensor 18 (as shown by dashed scan line 32').

Optical sensor 18 provides an error voltage when it intercepts a laserbeam that is offset from the physical center line of sensor 18. Whilevarious types of optical sensors may be employed for optical sensor 18,one that is preferred includes two light sensitive semiconductors thatare separated by a non-light-responsive area that is positioned directlyat the centerline of optical sensor 18. As a result, a change inposition of an incident laser beam from the centerline of optical sensor18 will cause one or the other of the adjacent photodetectors to providean output voltage, which output voltage is employed as an errorpotential to enable a positional correction voltage to be generated.

Positional correction of the laser beam is achieved by moving galvano 28to cause beam 26 to reposition itself with respect to rotating facetedmirror 30. In FIG. 2, a sectional view of galvano 28 is illustrated. Inessence, galvano 28 is a motor-driven mirror that is repositionableabout an axis that is perpendicular to the paper. Mirror 50 is mountedon a wire-wound rotor 52. A flexible mount 54 connects mirror 50/rotor52 to a magnetic return stator structure 56. A pair of magnets 58 and 60are positioned on stator structure 56 and about rotor 52. By appropriateenergization of rotor structure 52, mirror 50 may be caused to move inthe directions shown by arrows 62. As a result, laser beam 26 may bealtered in direction in accordance with the positioning of mirror 50which is, in turn, dependent upon the energizing current applied towire-wound rotor 52.

Turning to FIG. 3, a circuit is shown which enables banding reduction tobe accomplished through the use of the structure shown in FIGS. 1 and 2.It will be recalled that banding is the result of pitch errors betweensucceeding raster scan lines 40 (FIG. 1) which cause a series ofsucceeding scan lines to be either increasingly closer together orfarther apart, as the case may be.

The banding reduction control circuit comprises two control loops 70 and72, of which control loop 70 is a standard motor control servo looppresent in prior art laser printers. Control loop 72 implements theinvention in conjunction with control loop 70. A clock signal is appliedvia an input 74 to summers 76 and 77. Ideally, if the mechanical drivesystem for photoconductor belt 10 was "perfect", the positioning of eachscan line 40 in relation to clock signals applied to input 74 would beprecise and repeatable. However, due to the above-described mechanicalerrors, spacings between successive scan lines 40 or groups ofsuccessive scan lines can vary to cause the banding effect. The outputfrom summer 76 is applied through an integrator 78 and an amplifier 80to motor 82. An encoder 84 provides a signal train on feedback line 87that is indicative of succeeding instantaneous positions of motor 82.The difference between the feedback signals on line 87 and the inputclock signals applied via input 74 provides a motor error voltage whichis utilized to correct the speed of motor 82 in the known manner.Control loop 70 acts in a direction to drive the output of summer 76towards zero.

As motor 82 rotates, it operates a gear train 85 which, in turn, causesrotation of drive roller 16 and photoconductor belt 10. Theinstantaneous position of photoconductor belt 10 is sensed by an outputfrom position sensor 20 which, in this case, is a magnetic head. Thoseskilled in the art will fully understand that any appropriate positionalsensing system can be utilized to provide a pulse train that issynchronized with the movement of photoconductor 10 (e.g., optical,electrical or otherwise).

The pulse sequence output from position sensor 20 is fed via anamplifier 86 as an input to summer 77. The output from summer 77 (asintegrated by integrator 88) provides a photoconductor belt error signalthat includes a component which is directly related to any mechanicalpositioning errors. A positioning error causes a variation in anexpected time difference between pulses from position sensor 20 andclock pulses applied to summer 77. As a result, the output fromintegrator 88 is a photoconductor position error signal that is appliedto summer 90.

Summer 90 controls the operation of loop 72, which comprises amplifier92, galvano 28, laser beam 26 and optical sensor 18. The objective ofloop 72 is to adjust galvano 28 to reposition laser beam 26 (as sensedby optical sensor 18) in a direction to reduce the difference between afeedback voltage applied via line 94 to summer 90 and the photoconductorbelt error potential as applied to summer 90. More specifically, if thephotoconductor belt error signal, by its value, indicates thatphotoconductor 10 is lagging from where it should properly bepositioned, the voltage applied to summer 90 is increased, therebycausing a larger galvano error signal to be applied, via amplifier 92,to galvano 28. Mirror 50 (FIG. 2) is thus rotated to position laser beam26 to compensate for the positional lag of photoconductor 10. Galvano 28thus is caused to move scan line 40 in a direction to maintain thespacing from the immediately previous scan line at a constant value.More specifically, galvano 28 deflects laser beam 26 so as to intercepta facet on rotating faceted mirror 30 at a displaced position. Thatdisplaced position results in a movement of scan line 40 with respect tolaser write platen 16.

Because optical sensor 18 only provides an updated correction signalonce per scan, laser beam 26 is only repositioned once per scan. As aresult, optical sensor 18, as shown in FIG. 3, incorporates a voltageholding circuit that enables the maintenance of the feedback level online 94.

Turning to FIGS. 4 and 5, a further embodiment of the invention is shownwhich enables a beam position signal to be generated during a full scanand obviates the need for optical sensor 18 to include a voltage holdingcircuit. In FIG. 4, each element shown therein that is common to anelement shown in FIG. 1 is numbered identically. It is to be noted thata beam splitter 100 has been placed in the beam path between galvano 28and rotating faceted mirror 30. Beam splitter 100 directs beam 26towards a beam position sensor 102 which is constructed similarly tooptical sensor 18. Beam position sensor generates a continuous beamposition voltage. The output from beam position sensor 102 is employedin the control loop 72 shown in FIG. 5. Optical sensor 18 is stillemployed but provides its output directly to summer 90. Beam positionsensor 102 replaces optical sensor 18 in control loop 72 and enables acontinuous beam position voltage to be fed via line 94 to summer 90. Asbeam position sensor 102 is now located to provide a continuous beamposition feedback potential on line 94, feedback loop 72 need not awaitfor a change in output from optical sensor 18 to provide a correctionsignal to galvano 28.

The output from optical sensor 18, as applied to summer 90, enablessystem errors that occur between beam splitter 100 and scan line 40 tobe corrected. For instance, the output from optical sensor 18 will causean error voltage from summer 90 in the event anomalies are present inthe facets of rotating faceted mirror 30. Furthermore, optical sensor 18will also cause an error voltage should a vibrational mode occur in theoperating mechanism. Optical sensor 18 thus enables a periodic errorinput to supplement the error potential derived from beam positionsensor 102 and enables a more precise control of laser beam 26.

FIG. 6 is a side sectional view of a piezoelectric beam deflector 110.Piezoelectric beam deflector 110, is used in place of galvano 28 shownin FIGS. 1, 3, 4, and 5, in alternate embodiments of the presentinvention. Beam deflector 110 is of the type generally described as acantilever deflector and is preferred over alternate deflectors of thetype generally employing a voice coil motor piston or a piezoelectricpiston. The cantilever type deflector provides greater beam deflectioncapability over the piezoelectric piston type and provides more economiclife cycle costs when compared to deflectors of all other types.

Deflector 110 primarily includes support 112 and 114, bimorph crystalelement 120, and mirror 124, attached to element 120 by conventionaladhesive. Deflector 110 is supported from a chassis portion 111 of alaser printer, for example the laser printer shown in FIGS. 1 or 4.Support blocks 112 and 114 grip crystal element 120 and are held ontochassis 111 by one or more conventional fasteners or adhesives. Movementof crystal element 120 is substantially constrained by support blocks112 and 114 so that element 120 mechanically operates as a cantilever.

The structure and operation of crystal element 120 are of the typegenerally known as a bimorph, being constructed primarily of two wellknown ceramic materials 121 and 122. Crystal element 120 is of the typecurrently marketed for positioning systems and displacement transducers.Wires 126 and 128 apply a predetermined potential difference acrossmaterials 121 and 122, for example, causing material 121 to expand alongthe length of element 120 and the material 122 to contract. Reversingpolarity has the opposite effect on each material. Consequently, element120 bends and so moves mirror 124 to a particular position correspondingto the predetermined potential.

Accurate positioning of laser beam 26 is accomplished along the motiondirection indicated in FIG. 6. When element 120 is bent to orient mirror124 at a first position, laser beam 26 follows a facet path across afacet of rotating faceted mirror 30. Movement of photoconductor belt 10may be insufficient to place a subsequent scan line at a proper distancefrom a previous scan line, where the distance is measured between theparallel scan lines along the direction of paper movement through theprinter. In such a case, an appropriate error signal, for example asprovided by amplifier 92, causes element 120 to bend to a secondposition. The aforementioned error signal remains constant and element120, therefore, maintains the second position throughout the subsequentscan. Laser beam 26 is deflected by mirror 124 to follow a second facetpath across a facet of rotating faceted mirror 30. The second facet pathdiffers from the first facet path in axial position on a facet, whereaxial refers to a measurement along a line parallel to the axis ofrotation of mirror 30. Motion direction indicated by arrows 62,therefore, corresponds to the movement of paper through the printer asopposed to the direction of a scan line across the paper being printed.

Because element 120 and mirror 124 together have little mass as comparedto the moving portions of galvano 28, shown for example in FIG. 2,faster positioning of laser beam 26 by deflector 110 is accomplishedwith less power and without bearing surfaces that wear.

Deflector 110 is a type of absolute positioning component withelectromechanical properties sufficient, in several embodiments, foropen loop positioning applications. In an alternate embodiment derivedfrom the block diagram shown in FIG. 3, deflector 110 replaces galvano28; and, summer 90, cooperation for sensing beam 26, sensor 18, and line94 are omitted. In another alternate embodiment derived from the blockdiagram shown in FIG. 5, deflector 110 replaces galvano 28; and,cooperation for sensing beam 26, sensor 102, and line 94 are omitted.Control loop 70 remains for both derivative embodiments.

In all of the above described embodiments, repositioning laser scan line40 in accordance with photoconductor error positioning voltages enablesfeedback signals to be generated at electronic speed to correct formechanical anomalies that cause a mispositioning of photoconductor belt10. As a result, banding is minimized and higher quality graphics imagesare produced.

It should be understood that the foregoing description is onlyillustrative of the invention. Various alternatives and modificationscan be devised by those skilled in the art without departing from theinvention. For instance, while a motor structure has been shown forcontrolling galvano 28, piezoelectric control is equally applicable.Accordingly, the present invention is intended to embrace all suchalternatives, modifications and variances which fall within the scope ofthe appended claims.

What is claimed is:
 1. A print apparatus including a laser forgenerating a laser beam, a photoconductor and mechanical means formoving said photoconductor past a laser scan line exposure position, soas to enable a plurality of scan lines to expose said photoconductoralong a page movement direction, said print apparatus comprising:aphotoconductor position sensor for producing a photoconductor positionerror signal that is indicative of a mispositioning of saidphotoconductor; a circuit means for producing an error correction signalin response to said photoconductor position error signal; and apiezoelectric deflector responsive to said error correction signal tomove said laser beam so as to align a scan line on said photoconductorat a position characterized by a distance from a previous scan line onsaid photoconductor, said distance being measured along said pagemovement direction.
 2. The print apparatus as recited in claim 1 whereinsaid deflector comprises a mirror in mechanical communication with apiezoelectric bimorph element, wherein:said element bends in response tosaid error correction signal; and said mirror, being positioned tointercept said laser beam, moves said laser beam to align said scan linein response to said bending.
 3. The print apparatus as recited in claim2, wherein: said circuit further comprises a laser beam sensor forintercepting said laser beam and for providing a laser position errorsignal; and said circuit is further responsive to said laser positionerror signal to produce said error correction signal.
 4. The printapparatus as recited in claim 3 wherein said laser beam sensor isjuxtaposed to said laser scan line exposure position and produces saidlaser position error signal at least once per laser scan.
 5. The printapparatus as recited in claim 3 further comprising a rotating facetedmirror for scanning said laser beam across said laser scan line exposureposition and across said laser beam sensor, said laser beam sensor beingpositioned between said laser and said faceted mirror for producing saidlaser position error signal.
 6. The print apparatus as recited in claim5 wherein:the print apparatus further comprises a second laser beamsensor juxtaposed to said laser scan line exposure position forproducing a second position error signal: and said circuit is furtherresponsive to said second laser position error signal to produce saiderror correction signal.
 7. A print apparatus including aphotoconductor, and a mechanical drive system for moving saidphotoconductor past a scan line exposure station, said print apparatuscomprising:a reference signal source for producing a reference signal; asensor for producing a pulsed signal indicative of movement of saidphotoconductor; a first circuit for producing a position error signalderived from a difference between said reference signal and said pulsedsignal, said position error signal indicative of a position of saidphotoconductor which differs from a predetermined laser exposureposition determined with respect to said reference signal; a laser beamscanner for directing a laser beam in a scan line across saidphotoconductor; a beam detector for producing a beam position signal inresponse to an interception of said laser beam; a piezoelectric beamdeflector; and a second circuit responsive to said beam position signaland said position error signal said second circuit for producing adeflection control signal for said beam deflector, said deflectioncontrol signal causing movement in said beam deflect which movementcauses said laser beam to locate said scan line on said photoconductorat a position relative to the page movement direction and closer to saidpredetermined laser exposure position.
 8. The print apparatus as recitedin claim 7 wherein said piezoelectric beam deflector comprises:adeflection mirror for intercepting said laser beam; and a piezoelectricpositioner responsive to said beam deflection control signal tore-orient said deflection mirror so as to locate said scan line on saidphotoconductor.
 9. The print apparatus as recited in claim 8 whereinsaid laser beam scanner comprises a rotatable faceted mirror and alaser, said deflection mirror positioned in a beam path between saidrotatable faceted mirror and said laser.
 10. The print apparatus asrecited in claim 8 wherein said piezoelectric positioner comprises abimorph crystal element.
 11. The print apparatus as recited in claim 10wherein said bimorph crystal element comprises a cantilever beam. 12.The print apparatus as recited in claim 8 wherein said photoconductor isconfigured as an organic photoconductor belt which moves in contact witha laser write platen, said laser write platen supporting said beamdetector so that the beam position signal is produced at least once eachscan of said laser beam.
 13. The print apparatus as recited in claim 12,further comprising:a second beam detector positioned for, providing asecond beam position signal in accordance with varying beam positionduring a scan line.
 14. A scanning apparatus for improving laser printeroutput by reducing banding resulting primarily from invariant facet pathpositioning, said scanning apparatus comprising:a circuit comprising aphotoconductor position sensor and a laser beam position sensor, thecircuit for providing a signal in accordance with a misposition of thephotoconductor and in accordance with a misposition of a scan line; amirror comprising a plurality of facets, said mirror for rotation on anaxis to provide the scan line; and a deflector for receivingillumination from a provided laser and for illuminating a facet pathacross a facet of said plurality when said mirror rotates, wherein anaxial position of said facet path is variable in response to the signal.15. The scanning apparatus as recited in claim 14 wherein said deflectorcomprises a piezoelectric bimorph element that bends in response to saidsignal for determining said axial position.
 16. The scanning apparatusas recited in claim 15 wherein said piezoelectric bimorph elementcomprises a cantilever having a fixed end and a second end, a stationaryposition of said second end being determined by a bend in saidcantilever in response to said signal.
 17. The scanning apparatus asrecited in claim 16 wherein said cantilever comprises a mirror thatreceives illumination from said laser and illuminates said facet path.18. The scanning apparatus as recited in claim 17 wherein said mirror isfixed to said cantilever at said second end.
 19. The scanning apparatusas recited in claim 14 wherein said deflector comprises a motor.
 20. Thescanning apparatus as recited in claim 14 wherein said deflectorcomprises a piezoelectric piston.